Work apparatus having an internal combustion engine as a drive motor include a carburetor for generating an air/fuel mixture which is so adjusted that the air/fuel ratio is optimal for the running of the engine in the operationally warm condition. The work apparatus can include, for example, a motor-driven chain saw, a brushcutter, a cutoff machine or the like. For a cold start, however, a part of the fuel of the air/fuel mixture condenses on the cold walls of the engine and, as a consequence thereof, the air/fuel mixture is too lean. As a consequence, the engine does not start or starts only poorly.
A choke element is widely used for improving the cold start performance. With the choke element, the intake channel of the engine is partially closed in the region of the carburetor in such a manner that an enrichment of the air/fuel mixture occurs.
In a great many known carburetor configurations, a choke flap is integrated into the carburetor, which, however, requires a considerable constructive complexity. In other known configurations, a choke element is provided outside of the carburetor in the region of the intake opening in the air filter housing which lies upstream of the intake channel. This choke element, for example, can be pressed against the intake opening with a pivot arm. Here, a precise seating of the choke element on the intake opening is required for a precise reproducibility of the adjustment conditions for the cold start. The external arrangement of the choke element and especially its journaling, for example, on a wall of the forward-mounted air filter housing, can lead to tolerance problems. As a consequence of these tolerance problems, the choke element lies, in the closed state, with inadequate sealing in the region of the intake opening.
It is an object of the invention to provide a carburetor arrangement with improved insensitivity to tolerances in the region of the choke element.
The carburetor arrangement of the invention is for an internal combustion engine including an engine of a work apparatus including a motor-driven chain saw, brushcutter, cutoff machine or the like. The carburetor arrangement includes: a carburetor defining an intake channel; an air filter housing including an air filter arranged upstream of the intake channel; the air filter housing having a base wall defining an intake opening communicating with the intake channel; and, a choke assembly including: a carrier; a choke element arranged on the carrier so as to permit the choke element to at least partially cover the intake opening; and, means for holding the choke element so as to permit limited relative movement thereof.
In the above, the choke element is held on a separate choke carrier. The mounting of the choke carrier is designed for limited movement. The limiting of the movability leads to a coarse alignment of the choke element with respect to the intake opening. The mobility itself makes possible an automatic adaptation of the position of the choke element relative to the intake opening. As a practical matter, the movable mounting is so configured that the choke element is laterally displaceable on the choke carrier in a direction, which lies in the plane of the intake opening, and is laterally displaceable preferably in desired directions lying in the above-mentioned plane. Position tolerances, which lie laterally and in elevation, can, in this way, be compensated in a simple manner without a costly highly precise manufacture being necessary.
In an advantageous further embodiment of the invention, the choke element is pivotally held about one and preferably about any number of axes lying in the plane of the intake opening. In this way, angle tolerances between the plane of the choke element and the plane of the intake opening are automatically compensated. Accordingly, the complexity of an excessively high-precision manufacture is unnecessary.
In addition to the compensation of manufacturing tolerances, the above mentioned embodiments also easily make possible a compensation of, for example, thermal expansions, even, under some circumstances considerable, thermal expansion of an air filter housing, which is made of plastic and on which the choke carrier is journalled with the choke element.
In a practical further embodiment of the invention, a guard to prevent rotation or twisting of the choke element with respect to the longitudinal axis of the intake channel is provided. Especially in connection with aerodynamic ancillary devices (such as form bodies or air bores on the choke element), the alignment of the ancillary devices with respect to the carburetor is ensured.
The above-described connection of the choke element to the choke carrier provides for limited movement and is easily realizable by means of a snap connection having play. The constructive and manufacturing complexity is low and a limited movement in the above-described degrees of freedom is made possible in a simple manner in a corresponding configuration of the play. The snap connection additionally permits a rapid and cost-effective assembly.
In a constructively simple and effective embodiment of the snap connection, the choke element has a latch lug which can latch into a corresponding latch clamp of the choke carrier. A holding hook is advantageously provided on the latch lug and this holding hook engages behind an edge of the choke carrier in the assembled condition and thereby reliably prevents an axial slippage of the latch lug out of the latch clamp without affecting the assembly operation. The latch hook is advantageously so configured that it permits a limited axial play.
To simplify the assembly operation, the latch clamp has at least one assembly bevel whereby the latch clamp is automatically widened when the latch lug is introduced so that the latch lug can snap into the latch clamp. The above-described protection against rotation or twisting is constructively simply configured in that a holding lug is provided on the choke element at a radial spacing to the snap connection. The holding lug engages in a corresponding receptacle opening on the choke carrier.
To achieve a reliable and precise alignment of the choke element with respect to the intake opening to be covered, it is practical to provide a centering device by means of which the choke element is automatically aligned with respect to the intake direction. In a simple and effective embodiment, this automatic alignment is achieved in that a peripherally extending centering bevel, which is adapted to the contour of the intake opening, is provided on the choke element. The centering bevel can, for example, be in the form of a truncated cone. The peripherally extending centering bevel has an automatic centering action when the throttle element closes and furthermore forms a reliable self-adapting sealing seat for the choke element. In an advantageous variation, the intake opening has a rounded edge and the above-described centering bevel is configured as a concave chamfer. The rounded edge of the intake opening leads, in the opened state of the choke element, to a low intake flow resistance which contributes to improving the engine power. In the closed state of the choke flap, an areal contact engagement, and therefore a reliable seal, results from the connection of the rounded edge with the contact-engaging concave chamfer.
The choke carrier is advantageously configured as a pivot arm. With a simple pivot movement, the choke element is brought to the intake opening and only low tolerances must be compensated. The choke element is moved approximately in the direction of the channel axis especially with a pivot axis of the pivot arm which lies transversely to the longitudinal axis of the intake channel. In this way, a precise preadjustment is already provided and the centering bevel, which is described above, is cleanly introduced into the intake opening.
In a practical embodiment, the pivot arm is journalled in a wall of the air filter housing. This arrangement does not require a separate device for journaling and furthermore makes possible a common removal of the choke element with the air filter housing whereby the carburetor is, for example, exposed for service work with few hand motions.
The choke element is advantageously configured as an injection molded part and especially of plastic. In this way, complex shapes can be produced in a cost-effective manner under large-series manufacturing conditions, which, in combination with the tolerance insensitivity of the choke arrangement, leads to an excellent choke action in the closed state of the choke element and to a low intake resistance in the opened state.